1. Technical Field
The present invention generally relates to the packing of energy storage particles, for example activated carbon particles, as well as to compositions and devices containing such particles and methods related to the same.
2. Description of the Related Art
Energy storage materials are commonly employed in electrical storage and distribution devices. For example, devices containing particles of activated carbon, silicon, sulfur, lithium, and combinations thereof, as energy storage media are ubiquitous throughout the electrical industry. Of these, activated carbon particles find particular use in a number of devices because the high surface area, conductivity and porosity of activated carbon allows for the design of electrical devices having higher energy density than devices employing other materials.
Electric double-layer capacitors (EDLCs) are an example of devices that contain activated carbon particles. EDLCs often have electrodes prepared from an activated carbon material and a suitable electrolyte, and have an extremely high energy density compared to more common capacitors. Typical uses for EDLCs include energy storage and distribution in devices requiring short bursts of power for data transmissions, or peak-power functions such as wireless modems, mobile phones, digital cameras and other hand-held electronic devices. EDLCs are also commonly used in electric vehicles such as electric cars, trains, buses and the like.
Batteries are another common energy storage and distribution device which often contain activated carbon particles (e.g., as anode material, current collector, or conductivity enhancer). Examples of carbon-containing batteries include lithium air batteries, which use porous carbon as the current collector for the air electrode, and lead acid batteries which often include carbon additives in either the anode or cathode. Batteries are employed in any number of electronic devices requiring low current density electrical power (as compared to an EDLC's high current density).
An important characteristic to be considered in the design of electrical storage and distribution devices comprising activated carbon particles is volumetric performance. For example, in many of the devices described above, size is a constraint, and the physical size of the electrode is limited. Thus, high volumetric capacitance (i.e., capacitance per unit volume) is a desired characteristic of an electrode and the EDLC comprising the electrode(s). Volumetric capacitance of an EDLC is believed to be, at least in part, related to the efficiency of the activated carbon particle packing within the electrode. As the carbon particle packing approaches an optimum value (i.e., theoretical maximum number of carbon particles per unit volume), the inter-particle volume is minimized, and the volumetric capacitance of the EDLC electrode is expected to increase. This same principle is believed to apply to other types of energy storage particles and electrical devices containing the same.
Current methods for preparing activated carbon particles do not result in activated carbon particles having a particle size distribution which provides for optimized particle packing. One common method for producing high surface area activated carbon material is to pyrolyze an existing carbon-containing material (e.g., coconut fibers or tire rubber). Activated carbon materials can also be prepared by chemical activation. For example, treatment of a carbon-containing material with an acid, base or salt (e.g., phosphoric acid, potassium hydroxide, sodium hydroxide, zinc chloride, etc.) followed by heating results in an activated carbon material. Another approach for producing high surface area activated carbon materials is to prepare a synthetic polymer from carbon-containing organic building blocks. As with the existing organic materials, the synthetically prepared polymers are pyrolyzed and activated to produce an activated carbon material. In contrast to the traditional approach described above, the intrinsic porosity of the synthetically prepared polymer results in higher process yields because less material is lost during the activation step.
The activated carbon particles prepared according to the above methods may be further processed to reduce the particle size. Such methods include milling, such as ball milling, cryo-milling and bead milling, as well as crushing. While these methods may improve the particle packing over the unprocessed carbon material, current applications of such methods are not sufficient to provide an activated carbon material having a particle size distribution which provides for optimized particle packing.
While significant advances have been made in the field, there continues to be a need in the art for energy storage materials, for example activated carbon particles, comprising a particle size distribution which provides for optimized particle packing, as well as for methods of making the same and devices containing the same. The present invention fulfills these needs and provides further related advantages.